1. Field of the Invention
This invention relates to sewing and specifically to a sewing machine having a fabric-edge tracing function.
2. Description of the Prior Art
Fabric-edge tracing sewing machines are already known wherein stitches are formed along a line spaced by a predetermined distance from an edge of a workpiece or work fabric.
One of such fabric-edge tracing sewing machines is disclosed, for example, in Japanese Patent Laid-Open No. 221389/1987 filed by the assignee of the present patent application, wherein a fabric-edge sensor is moved by a stepping motor in a lateral direction so that the amount of light received by a light receiving element provided on the fabric-edge sensor may be equal to a preset value, that is, a value which is presented when an edge of a work fabric is detected, and a needle bar is swung to a needle location which is determined in accordance with a preset tracing width and a position signal which is generated from a fabric-edge position detector in response to movement of the fabric-edge sensor.
Another tracing sewing machine has been put into practical use wherein such a fabric-edge sensor as described above is provided on a needle bar and the needle bar is swung by means of a needle bar swinging stepping motor so that the amount of light received by the light receiving element of the fabric-edge sensor may present a preset value when a needle is positioned above a face of a bed.
Stepping motors are used for fabric edge sensor drives and for needle bar swinging drives because they do not need the relatively complex positional control systems required for use of DC motors in such applications. Thus, stepping motors are used because they are more space and cost effective. However, stepping motors have a slow response time compared with DC motors.
In the tracing sewing machine disclosed in Japanese Patent Laid-Open No. 221389/1987 mentioned hereinabove, when the work fabric is moved by a great distance in a lateral direction while tracing sewing is being performed, or when the position of an edge of the work fabric varies to a great extent in accordance with a profile of the work fabric, or else when the work fabric is turned in order to perform tracing sewing at a corner of the work fabric or in a like case, the relative position between the fabric-edge sensor and the edge of the work fabric varies to a great extent, which increases the period of time required for the movement of the fabric-edge sensor. Accordingly, the fabric-edge sensor may fail to detect the edge of the work fabric accurately by a next needle position calculation timing, and since a needle location is determined in accordance with artificial needle position data which is based on current data during movement of the fabric-edge sensor and needle position data one stitch cycle ago to perform tracing sewing, there is a problem that the quality in tracing sewing is deteriorated.
Further, in the tracing sewing machine which provides the fabric-edge sensor on the needle bar, since an edge of a work fabric is detected only when the needle is positioned above the face of the bed, the period permitting the movement of the fabric-edge sensor is further decreased, and consequently, there is a problem that the quality of tracing sewing is further deteriorated.